[TX '90 ZR1]

If you still have the original cap on your fill tank(or even if not original) get it tested. On my '90 I found the pressure it would hold was considerably lower than it should be. This is probably more a function of age than miles. Mine has 34K.
I think the original spec on the cap is 17 lbs, but a couple of the members here recommended a 15 lb cap so I went with that.
These systems are getting some age on them & don't want to push the system!